Prior obstacles to the immunoprecipitation of purified rabbit reticulocyte polysomes through the successive use of (a) RNase-free goat-anti-alpha or anti-beta-chain, i.e., IgG molecules reactive with polysome-bound nascent globin chains, and (b) RNase-free donkey-anti- goat-IgG (molar ratios employed were polysomes: anti-chain: anti-IgG, 1:5:50) have been circumvented. The key ingredient is rabbit hemoglobin itself which, in final concentrations of greater than 100 mg/ml, lessens non-specific immunological entrappment of unwanted polysomes to negligible levels. Thus, for example, the purification via immunoprecipitation at 4 degrees of hemoglobin-alpha-chain polysomes -- assessed by C14 ratios obtained from nascent chains previously labeled with (H3)-isoleucine and (C14)-valine -- is essentially complete, i.e., free of hemoglobin-beta-polysomes, approximately 2 hours after beginning immune reactions; such immunoprecipitates, when washed, contain approximately 1/3 to 2/3 of all alpha-polysomes initially present. The high concentrations of hemoglobin used non-immunologically act on the reaction between goat-IgG and donkey-anti-goat-IgG. Hemoglobin also serves to lessen the spontaneous aggregation of reticulocyte polysomes which occurs in the absence of immunological reactions. The method lends itself to the isolation of nearly milligram quantities of largely pure alpha-mRNA or beta-mRNA.